Do trophic relationships in soil enhance organic P cycling and plant P nutrition? Phytate mineralization as a case study

Abstract

Phosphorus (P) strongly limits plant productivity as plants can only absorb free inorganic orthophosphate (Pi) at very low concentrations in soil solution. However, soil contains high levels of poorly available P, especially phytate, considered as the most abundant plant-unavailable organic P source. Here, we investigated a new strategy based on rhizosphere trophic relationships to mobilize P from phytate. We hypothesized that the interactions between plant (Pinus pinaster), phytase-producing bacteria (Bacillus subtilis), mycorrhizal fungi (Hebeloma cylindrosporum), representing the more widespread strategy to improve plant P acquisition, and bacterial grazer nematodes (Rhabditis sp.) may improve plant P acquisition from phytate and thus P cycling from soil organic P. We grew seedlings jn microcosms containing soil with Pi or phytate, with or without the above-mentioned organisms for 2.5 months. With Pi, no significant differences were observed among inoculation or mycorrhizal treatments. In contrast, with phytate, nematode grazing was required for non-mycorrhizal plants to acquire P into their shoots. In mycorrhizal plants, bacteria alone improved net P accumulation and nematode grazing enhanced this positive effect. Soil microbial P contents and in situ probing of bacterial phytase gene expression are currently under study to understand better the mechanisms underlying the strong positive effects of nematode grazing on plant P nutrition. Our first results indicate that the use of trophic relationships should be considered as a sustainable strategy for plant P nutrition to enhance organic P cycling and P availability in the rhizosphere